In the manufacture and sale of thermoplastic polymers, it is customary to introduce various additives into the polymer before such polymers are sold or used. Specifically, such additives are incorporated to stabilize the polymer against oxidation, deterioration from exposure to ultraviolet light, or to modify the tendency of films prepared from such polymers to stick to each other or "block." Many of the additives employed for this purpose are finely-divided solids. Polymers containing such additives frequently are referred to as compounded polymers.
The incorporation of additives into thermoplastic polymers presents greater difficulties than would be expected upon first consideration. This results from the fact that most additives are employed in low concentrations. In addition, many additives, particularly finely-divided solids, are not readily "wetted" and dispersed by highly viscous molten polymers. To overcome and/or minimize such problems, the art frequently uses master-batching techniques in which high levels of additives are incorporated into a polymer, which then is blended with additional "virgin" polymer to prepared compounded polymers having the desired concentration of the additive. The technique is used only out of necessity as it adds to the cost of preparing the desired compounded polymers. In addition, masterbatching introduces into the finished polymer an aliquot of polymer having an undesirably long heat history which adversely modifies certain polymer properties.
Certain thermoplastic polymers, particularly low density ethylene polymers and styrene polymers prepared by continuous mass polymerization processes, are discharged from the polymerization reaction zone in a molten state. The art has long recognized that the cost of manufacturing such polymers having additives dispersed therein could be reduced significantly if the desired additive(s) could be incorporated into the molten polymer, either as, or immediately after, the molten polymer is discharged from the polymerization zone. To date, the art has not developed satisfactory methods for incorporating additive(s) into such molten polymers at this point of their manufacture.
In view of the situations discussed above, there is a need in the art for developing an improved method for incorporating additive(s) into thermoplastic polymers.